The measurement of open channel flow in municipal wastewater collection systems is important to protect public health, municipal infrastructure, and the environment. Raw (untreated) drinking water, irrigation water, and plant effluent water are also transported via engineered open channels and pose similar measurement challenges. Accurate flow metering is necessary for billing, engineering studies, mitigation of unwanted inflow and infiltration, and for the control of the actual flow itself. Flow volumes must be understood and managed to minimize the impact of peak flows on wastewater treatment facilities and to reduce the possibility of untreated sewage reaching the environment.
There are a number of open channel flow meters in existence today. For example, there is a flow meter for measuring both the fluid velocity and the fluid level by non-invasive level sensors and velocity sensors that can be mounted inside a manhole above the flowing channel (typically just above the top of the pipe or culvert, a crown mount). By way of example, an energy beam may be directed toward the surface of a fluid flowing in an open channel. The beam may comprise directed waves of electromagnetic or acoustic energy. Examples of electromagnetic beams include radar and laser beams. A typical acoustic beam may utilize directed ultrasonic energy. Any such beam may be considered to transmit an oscillatory signal, characterized by a particular oscillation frequency or spectrum of frequencies. In each case, the energy beam is directed along a line toward the fluid surface and at least a portion of the transmitted energy is reflected from the fluid surface and subsequently received by the sensor. This reflected energy is then analyzed as a signal carrying meaningful information. In particular, the Doppler frequency shift between the directed and reflected signals is used as a measure of the velocity of the fluid surface. Typically, a non-invasive measure of the fluid depth is also obtained, e.g., using downward-looking ultrasonic, microwave, or other measurement techniques. Often, what is measured is the time delay between a transmitted energy pulse and its subsequent reflection from the fluid surface. From this delay, the vertical distance (“range”) between the sensor and the fluid surface is determined. Prior knowledge of the sensor mounting position relative to the channel bottom allows conversion from range to fluid depth. Finally, by combining the determined velocity and depth with prior knowledge about the cross-sectional shape of the channel, the volumetric flow rate of the fluid is calculated.